Yuchen Guo

Yuchen Guo

A PhD candidate at Department of Physics, Tsinghua University

Talks and presentations

Strong-to-weak spontaneous symmetry breaking meets average symmetry-protected topological order

March 18, 2025

Talk, APS March Meeting, Anaheim Convention Center: 253A

Recent studies have unveiled new possibilities for discovering intrinsic quantum phases that are unique to open systems, including phases with average symmetry-protected topological (ASPT) order and strong-to-weak spontaneous symmetry breaking (SWSSB) order in systems with global symmetry. In this work, we propose a new class of phases, termed the double ASPT phase, which emerges from a nontrivial extension of these two orders. This new phase is absent from prior studies and cannot exist in conventional closed systems. Using the recently developed imaginary-Lindbladian formalism, we explore the phase diagram of a one-dimensional open system with Z2×Z2 symmetry. We identify universal critical behaviors along each critical line and observe the emergence of an intermediate phase that completely breaks the Z2 symmetry, leading to the formation of two triple points in the phase diagram. These two triple points are topologically distinct and connected by a domain-wall decoration duality map. Our results promote the establishment of a complete classification for quantum phases in open systems with various symmetry conditions.

A New Framework for Quantum Phases in Open Systems: Steady State of Imaginary-Time Lindbladian Evolution

October 17, 2024

Talk, Koushare, Online

In this talk, I plan to introduce our two recent works (arXiv 2403.16978 and arXiv 2408.03239) on topological phases in open quantum systems. I will begin with reviewing the classification of quantum phases in closed systems and properties of corresponding phase transitions. Next, I will discuss our tensor network construction for a specific class of topological states, namely average symmetry protected topological (ASPT) phases, defined in open systems, especially with a nontrivial extension of strong and weak symmetry. I will then introduce a new framework to systematically study the phase transitions between different open system quantum phases with the imaginary-time Lindbladian evolution. To illustrate the effectiveness of this framework, we apply it to investigate the phase diagram for open systems with Z_2^σ×Z_2^τ symmetry, including cases with nontrivial ASPT order or spontaneous symmetry breaking (SSB) order. I will finally discuss several universal properties at quantum criticality, such as nonanalytic behaviors of steady-state observables, divergence of correlation lengths, and closing of the imaginary-Liouville gap. These results advance our understanding of quantum phase transitions in open quantum systems.

Non-Hermitian Parent Hamiltonian and Composite Quantum Phases

April 27, 2023

Talk, Koushare, Online

In this talk, I plan to introduce our two recent works (arXiv: 2301.12448, arXiv: 2304.04588) on non-Hermitian interacting spin systems. I will begin with introducing the basic concept of tensor networks, and the conventional parent Hamiltonian method for Hermitian systems. Next, I will discuss our proposed non-Hermitian parent Hamiltonian (nH-PH), where one can start from two different matrix product states (MPS) and construct a local Hamiltonian such that these states are zero-energy modes. I will then introduce a class of new topological phases in non-Hermitian interacting systems without Hermitian counterparts, denoted as composite quantum phases. With the nH-PH approach, we construct a spin-1 model to realize this type of exotic phases. I will finally show the phase diagram of our model and demonstrate that such novel phases can exist extensively in non-Hermitian systems.

Quantum noise effects and quantum error mitigation

November 02, 2022

Talk, Institute for Interdisciplinary Information Sciences, Tsinghua University, MMW s327

In this talk, I plan to introduce our two recent works (arXiv: 2201.00752, arXiv: 2207.01403). I will begin with briefly introducing the basic concepts of quantum error mitigation (QEM) in the era of noisy intermediate-scale quantum (NISQ) devices and a typical QEM approach, namely the quasi-probability decomposition method. I will then make comments on this method from two aspects, i.e. its powerlessness on correlated noise and the physical meaning of its sampling cost, respectively. In the remaining time, I will focus on these two issues. In our first work (arXiv: 2201.00752), we use matrix product operators (MPO) to represent the noise channel to characterize and mitigate correlated noise in quantum circuits. As for the second work (arXiv:2207.01403), we find that the physical implementability, which is the sampling cost of implementing the noise inverse, is a good characterization of the decoherence effects.